The three principal components of biomass are cellulose, lignin and hemicellulose, and they are present in almost all plant cell walls. The cellulosic material obtained from such biomass has a number of important industrial uses, notably in the production of paper from wood pulp. Accordingly a variety of processes have been developed for treating biomass to separate cellulosic material from other components of biomass, including the Kraft and sulfite processes. As well as producing cellulosic wood pulp, those processes also result in the production of by-products known as black liquor (Kraft process) or brown liquor (sulfite process) which typically contain hemicellulosic material together with lignin/lignin-derived products and inorganic chemicals. In recent times, demand for wood pulp containing higher cellulose content has been increasing, and processes for producing such forms of wood pulp (known as “dissolving pulp” or “dissolving cellulose”) have been developed. Dissolving pulp finds use in the production of products such as rayon, viscose and cellophane. Typically in a process for producing dissolving pulp, an additional “pre-hydrolysis” stage is carried out in which lignocellulosic biomass is treated to remove hemicellulosic material and lignin/lignin-derived products, prior to subjecting the remainder of the cellulosic solids to further pulping conditions, such as Kraft conditions (treatment with an aqueous solution of sodium hydroxide and sodium sulphide at elevated temperature) or sulfite conditions (treatment with aqueous metal sulfite and/or bisulfite at elevated temperature). The separated hemicellulosic stream obtained from processes for producing dissolving pulp is typically referred to as “pre-hydrolysate liquor” or “pre-hydrolysis liquor”, PHL.
Whilst the cellulosic material obtained from processes such as those outlined above is taken on and processed into various useful products, the hemicellulosic streams are often considered to be of little value and may be burnt or fed into a gasifier to recover their energy value.
A further, more recent, example of the use of cellulosic material as a feedstock in industrial processes is in the field of bioethanol production. Processes for the production of those biofuels from crop sources such as sugar beet and sweet sorghum have been developed and refined, and there has been a significant rate of growth in biofuel production in recent years. However, the identification of suitable feedstocks for the production of chemicals can be complicated. For instance, the increase in biofuel production has led to competition for crops, crop-switching and price increases for food products. Typically, the first stages of cellulosic ethanol production involve pre-treatment of biomass and removal of the hemicellulosic fraction, with the separated cellulosic material subsequently being converted into bioethanol by hydrolysis to glucose and subsequent fermentation to ethanol. The use of hemicellulosic material as a feedstock for bioethanol production has also been investigated. However, the effectiveness of fermentation of hemicellulosic feedstocks is limited by the fact that unlike cellulosic feedstocks they contain a mixture of pentose and hexose sugars which is typically harder for microorganisms to utilise. Additionally, hemicellulosic feedstocks tend to contain acids, aldehydes, furan derivatives and lignin-derived products which can act to inhibit the effectiveness of fermentation-utilising processes, due to the sensitivity of the species of microorganisms typically employed to those compounds. As a result, hemicelluloses currently represent the largest polysaccharide fraction wasted in most cellulosic ethanol pilot and demonstration plants around the world (Girio et al, Bioresource Technology, 2010, 101 p 4775-4800).
One well-known method of treating biomass is the “organosolv” process. This process involves treating whole biomass with one of a variety of organic solvents, for example an alcohol, and the result of the process is generally three product streams: cellulose, hemicellulose and lignin. Much research has been carried out to determine suitable conditions under which the organosolv process can be performed to optimise yields of the desired products. For example, Wang et al, Process Biochem. 47 (2012) 1503-1500, describes a series of experiments to determine the effects of varying catalysts and solvents in the organosolv process, producing “useful data for the application of mild organosolv fractionation on the utilization of whole biomass, especially for the recovery of hemicellulosic components.”
Hiujgen et al, Bioresource Tech. 114 (2012) 389-398 describes a development of the organosolv process in which the biomass is pretreated in order to hydrolyse part of the biomass, specifically the hemicellulose, into sugars. These sugars are removed as part of a liquid stream, leaving a wet pulp stream which is relatively rich in cellulose and lignin (the hemicellulose having been degraded and removed), and which is subsequently treated using the organosolv process to remove the lignin from the cellulose.
Lactic acid (2-hydroxypropanoic acid) and its cyclic dimer lactide (3,6-dimethyl-1,4-dioxan-2,5-dione) are important building blocks for the chemical and pharmaceutical industries. One example of their use is in the manufacture of polylactic acid, the biodegradability of which makes it an attractive candidate to replace more conventional polymers. A number of processes are known for producing lactic acid, including chemical synthesis and fermentation methods. According to Boudrant et al, Process Biochem. 40 (2005) p. 1642, “In 1987, the world production of lactic acid averaged approximately equal proportions being produced by chemical synthesis and fermentation processes”. Such chemical syntheses typically employed the hydrocyanation of acetaldehyde. However, chemical processes of this type have long been regarded as inefficient on an industrial scale, and today virtually all large scale production of the lactic acid available commercially is manufactured by fermentation processes, see for example Strategic Analysis of the Worldwide Market for Biorenewable Chemicals M2F2-39, Frost and Sullivan, 2009. In a typical fermentation process, glucose is fermented by microorganisms to produce either D- or L-lactic acid, predominantly L-lactic acid. Companies such as Cargill and Corbion (formerly Purac) operate large-scale fermentation processes for the production of optically active lactic acid, and the patent literature is replete with improvements in such processes.
WO2012/052703 discloses a process for producing a complex of lactic acid and either ammonia or an amine, comprising reaction of one or more saccharides with barium hydroxide to produce a first reaction mixture comprising barium lactate, and contacting at least part of the first reaction mixture with ammonia or an amine and with carbon dioxide, or with the carbonate and/or bicarbonate salt of ammonia or an amine, to produce a second reaction mixture comprising the complex and barium carbonate. WO2012/052703 recommends the use of cellulose or starch as feedstocks. In particular, it refers to the use of invert sugar, or of glucose obtained from enzymatic hydrolysis of starch contained in biomass feedstocks such as maize, rice or potatoes. There is no mention of the possible use of hemicellulose.
Constituents of hemicellulose typically have a significant degree of acylation. For example, xylan hemicellulose is often found in a highly acetylated form. Conditions used for separating hemicellulosic material from cellulosic material typically result in significant quantities of organic acids (e.g. acetic acid, formic acid) being formed in the hemicellulosic stream. Accordingly, such streams would not be expected to be good feedstocks for production of lactic acid, due to contamination of the lactic acid product with other organic acids which may be difficult to separate.
We have now found an improved method of optimising the yields of useful products obtained from processes for the treatment of biomass. Specifically, we have found a process which allows the biomass to be treated in a first, conventional, step, to produce a hemicellulosic stream, and the hemicellulosic stream to be treated in subsequent steps to produce useful products. This invention gives efficient lignin removal from lignin-containing hemicellulosic streams and leads to high yields of hemicellulose-derived monosaccharides, which in turn leads to the unexpected result that hemicellulosic streams can be used inter alia as viable feedstocks for the production of lactate-containing species, chemical intermediates useful in the biopolymer and other industries. In particular, process conditions have been identified which enable metal lactate or quaternary ammonium lactate to be obtained in surprisingly good purity and in viable yield for use on an industrial scale.